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Types

Endocrine glands â€” are glands that secrete their products through the basal lamina and lack a duct system.

Exocrine glands â€” secrete their products through a duct or directly onto the apical surface, the glands in this group can be divided into three groups:

Apocrine glands â€” a portion of the secreting cell's body is lost during secretion. Apocrine gland is often used to refer to the apocrine sweat glands, however it is thought that apocrine sweat glands may not be true apocrine glands as they may not use the apocrine method of secretion.

Sebaceous glands â€” secrete a lipid product.These glands are also known as oil glands.

Formation

Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may from the beginning possess a tubular structure, but in other instances glands may start as a solid column of cells which subsequently becomes tubulated.

As growth proceeds, the column of cells may divide or give off offshoots, in which case a compound gland is formed. In many glands the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sub-division. As a rule, the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In compound glands the more typical or secretory epithelium is found forming the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.

Glands are classified according to their shape.

If the gland retains its shape as a tube throughout it is termed a tubular gland.

In the second main variety of gland the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar or saccular glands.

Additional images

The lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph (from Latin lympha "water"[1]) unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils. The system also includes all the structures dedicated to the circulation and production of lymphocytes, which includes the spleen, thymus, bone marrow and the lymphoid tissue associated with the digestive system.[2] The lymphatic system as we know it today was first described independently by Olaus Rudbeck and Thomas Bartholin.

The blood does not directly come in contact with the parenchymalcells and tissues in the body, but constituents of the blood first exit the microvascular exchange blood vessels to become interstitial fluid, which comes into contact with the parenchymal cells of the body. Lymph is the fluid that is formed when interstitial fluid enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network by either intrinsic contractions of the lymphatic passages or by extrinsic compression of the lymphatic vessels via external tissue forces (e.g. the contractions of skeletal muscles).

The lymph transports antigen-presenting cells (APCs), such as dendritic cells, to the lymph nodes where an immune response is stimulated.

Lymphatic tissue is a specialized connective tissue - reticular connective, that contains large quantities of lymphocytes.

Clinical significance

The study of lymphatic drainage of various organs is important in diagnosis, prognosis, and treatment of cancer. The lymphatic system, because of its physical proximity to many tissues of the body, is responsible for carrying cancerous cells between the various parts of the body in a process called metastasis. The intervening lymph nodes can trap the cancer cells. If they are not successful in destroying the cancer cells the nodes may become sites of secondary tumors.

Organization

The lymphoid system can be broadly divided into the conducting system and the lymphoid tissue.

The lymphoid tissue is primarily involved in immune responses and consists of lymphocytes[4] and other white blood cells enmeshed in connective tissue through which the lymph passes. Regions of the lymphoid tissue that are densely packed with lymphocytes are known as lymphoid follicles. Lymphoid tissue can either be structurally well organized as lymph nodes or may consist of loosely organized lymphoid follicles known as the mucosa-associated lymphoid tissue (MALT)

Lymphoid tissue

Lymphoid tissue associated with the lymphatic system is concerned with immune functions in defending the body against the infections and spread of tumors. It consists of connective tissue with various types of white blood cells enmeshed in it, most numerous being the lymphocytes.

The lymphoid tissue may be primary, secondary, or tertiary depending upon the stage of lymphocyte development and maturation it is involved in. (The tertiary lymphoid tissue typically contains far fewer lymphocytes, and assumes an immune role only when challenged with antigens that result in inflammation. It achieves this by importing the lymphocytes from blood and lymph.[5])

Primary lymphoid organs

The thymus and the bone marrow constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

Secondary lymphoid organs

Secondary or peripheral lymphoid organs maintain mature naive lymphocytes and initiate an adaptive immune response. The peripheral lymphoid organs are the sites of lymphocyte activation by antigen. Activation leads to clonal expansion and affinity maturation. Mature lymphocytes recirculate between the blood and the peripheral lymphoid organs until they encounter their specific antigen.

Lymph nodes

A lymph node is an organized collection of lymphoid tissue, through which the lymph passes on its way to returning to the blood. Lymph nodes are located at intervals along the lymphatic system. Several afferent lymph vessels bring in lymph, which percolates through the substance of the lymph node, and is drained out by an efferent lymph vessel.

The substance of a lymph node consists of lymphoid follicles in the outer portion called the "cortex", which contains the lymphoid follicles, and an inner portion called "medulla", which is surrounded by the cortex on all sides except for a portion known as the "hilum". The hilum presents as a depression on the surface of the lymph node, which makes the otherwise spherical or ovoid lymph node bean-shaped. The efferent lymph vessel directly emerges from the lymph node here. The arteries and veins supplying the lymph node with blood enter and exit through the hilum.

Lymph follicles are a dense collection of lymphocytes, the number, size and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly upon encountering a foreign antigen. The selection of B cells occurs in the germinal center of the lymph nodes.

Lymph nodes are particularly numerous in the mediastinum in the chest, neck, pelvis, axilla (armpit), inguinal (groin) region, and in association with the blood vessels of the intestines.[2]

Tubular vessels transport back lymph to the blood ultimately replacing the volume lost from the blood during the formation of the interstitial fluid. These channels are the lymphatic channels or simply called lymphatics.[6]

Function of the fatty acid transport system

Lymph vessels called lacteals are present in the lining of the gastrointestinal tract, predominantly in the small intestine. While most other nutrients absorbed by the small intestine are passed on to the portal venous system to drain via the portal vein into the liver for processing, fats (lipids) are passed on to the lymphatic system to be transported to the blood circulation via the thoracic duct. (There are exceptions, for example medium-chain triglycerides (MCTs) are fatty acid esters of glycerol that passively diffuse from the GI tract to the portal system.) The enriched lymph originating in the lymphatics of the small intestine is called chyle. As the blood circulates, fluid leaks out into the body tissues. This fluid is important because it carries food to the cells and waste back to the bloodstream. The nutrients that are released to the circulatory system are processed by the liver, having passed through the systemic circulation. The lymph system is a one-way system, transporting interstitial fluid back to blood.

Diseases of the lymphatic system

Lymphedema is the swelling caused by the accumulation of lymph fluid,[7] which may occur if the lymphatic system is damaged or has malformations. It usually affects the limbs, though face, neck and abdomen may also be affected.

Lymphatic tissues begin to develop by the end of the fifth week of embryonic development. Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm.

The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, upper limbs, neck and head. Some of the plexuses enlarge and form lymphatic vessels in their respective regions. Each jugular lymph sac retains at least one connection with its jugular vein, the left one developing into the superior portion of the thoracic duct.

The next lymph sac to appear is the unpaired retroperitoneal lymph sac at the root of the mesentery of the intestine. It develops from the primitive vena cava and mesonephric veins. Capillary plexuses and lymphatic vessels spread from the retroperitoneal lymph sac to the abdominal viscera and diaphragm. The sac establishes connections with the cisterna chyli but loses its connections with neighboring veins.

The last of the lymph sacs, the paired posterior lymph sacs, develop from the iliac veins. The posterior lymph sacs produce capillary plexuses and lymphatic vessels of the abdominal wall, pelvic region, and lower limbs. The posterior lymph sacs join the cisterna chyli and lose their connections with adjacent veins.

With the exception of the anterior part of the sac from which the cisterna chyli develops, all lymph sacs become invaded by mesenchymal cells and are converted into groups of lymph nodes.

The spleen develops from mesenchymal cells between layers of the dorsal mesentery of the stomach. The thymus arises as an outgrowth of the third pharyngeal pouch.

Lymphatico-Venous Communications

Present research has found cues about a lymphatico-venous communication. In mammals, lymphatico-venous communications other than those at the base of the neck are not easy to demonstrate, but described in some experiments.[8]

The specialists observed that the pulmonary complications following lymphangiography (a test which utilizes X ray technology, along with the injection of a contrast agent, to view lymphatic circulation and lymph nodes for diagnostic purposes)are more often severe in patients with lymphatic obstruction. In these cases, the contrast medium is thought to reach the vascular system via lymphovenous communications with shunt the material directly into the venous stream, bypassing those lymph nodes distal to the communications,[9] Because less contrast agent is absorbed in lymph nodes, a greater portion of the injected volume passes into the vascular system. Since pulmonary complications are related to the amount of medium reaching the lungs area, the early recognition of lymphovenous communications is a great significance to the lymphangiographer.[9] Another "hint" in proving a lymph-vein communication is offered by a Robert F Dunn experiment. The passage of radioactively tagged tracers, injected at elevated pressure, through the lymph node-venous communications coincides with the increased pressures of injection and subsequent nodal palpation in dogs. The passage of iodinated I 125 serum albumen (ISA) indicates that direct lymph node-venous communications are present, whereas passage of nucleated erythrocytes requires a communication structure the size of a capillary or larger.[10] Moreover, the evidence suggest that in mammals under normal conditions, mostly of the lymph is returned to the blood stream through the lymphatico-venous communications at the base of the neck. When the thoracic duct-venous communication is blocked, however, the resultant raised intralymphatic pressure will usually cause other normal non-functioning communications to open and thereby allow the return of lymph to the blood stream.[8]

History

Hippocrates was one of the first persons to mention the lymphatic system in 5th century BC. In his work "On Joints," he briefly mentioned the lymph nodes in one sentence. Rufus of Ephesus, a Roman physician, identified the axillary, inguinal and mesenteric lymph nodes as well as the thymus during the 1st to 2nd century AD.[11] The first mention of lymphatic vessels was in 3rd century BC by Herophilos, a Greek anatomist living in Alexandria, who incorrectly concluded that the "absorptive veins of the lymphatics", by which he meant the lacteals (lymph vessels of the intestines), drained into the hepatic portal veins, and thus into the liver.[11] Findings of Ruphus and Herophilos findings were further propagated by the Greek physician Galen, who described the lacteals and mesenteric lymph nodes which he observed in his dissection of apes and pigs in the 2nd century AD.[11][12]

Until the 17th century, ideas of Galen were most prevalent. Accordingly, it was believed that the blood was produced by the liver from chyle contaminated with ailments by the intestine and stomach, to which various spirits were added by other organs, and that this blood was consumed by all the organs of the body. This theory required that the blood be consumed and produced many times over. His ideas had remained unchallenged until the 17th century, and even then were defended by some physicians.[12]

In the mid 16th century Gabriele Falloppio (discoverer of the fallopian tubes) described what are now known as the lacteals as "coursing over the intestines full of yellow matter."[11] In about 1563 Bartolomeo Eustachi, a professor of anatomy, described the thoracic duct in horses as vena alba thoracis.[11] The next breakthrough came when in 1622 a physician, Gaspare Aselli, identified lymphatic vessels of the intestines in dogs and termed them venae alba et lacteae, which is now known as simply the lacteals. The lacteals were termed the fourth kind of vessels (the other three being the artery, vein and nerve, which was then believed to be a type of vessel), and disproved Galen's assertion that chyle was carried by the veins. But, he still believed that the lacteals carried the chyle to the liver (as taught by Galen).[13] He also identified the thoracic duct but failed to notice its connection with the lacteals.[11] This connection was established by Jean Pecquet in 1651, who found a white fluid mixing with blood in a dog's heart. He suspected that fluid to be chyle as its flow increased when abdominal pressure was applied. He traced this fluid to the thoracic duct, which he then followed to a chyle-filled sac he called the chyli receptaculum, which is now known as the cisternae chyli; further investigations led him to find that lacteals' contents enter the venous system via the thoracic duct.[11][13] Thus, it was proven convincingly that the lacteals did not terminate in the liver, thus disproving Galen's second idea: that the chyle flowed to the liver.[13]Johann Veslingius drew the earliest sketches of the lacteals in humans in 1647.[12]

Thomas Bartholin

The idea that blood recirculates through the body rather than being produced anew by the liver and the heart was first accepted as a result of works of William Harveyâ€”a work he published in 1628. In 1652, Olaus Rudbeck (1630â€“1702), a Swede, discovered certain transparent vessels in the liver that contained clear fluid (and not white), and thus named them hepatico-aqueous vessels. He also learned that they emptied into the thoracic duct, and that they had valves.[13] He announced his findings in the court of Queen Christina of Sweden, but did not publish his findings for a year,[14] and in the interim similar findings were published by Thomas Bartholin, who additionally published that such vessels are present everywhere in the body, and not just the liver. He is also the one to have named them "lymphatic vessels".[13] This had resulted in a bitter dispute between one of Bartholin's pupils, Martin Bogdan,[15] and Rudbeck, whom he accused of plagiarism.[14]

"The biology of the lymphatic system is positioned at a unique locus where the working of the immune system, its pivotal role in defending against cancer and infectious diseases, and its potential role in the treatment of autoimmunity uniquely align themselves. Many diseases will benefit from the advancement of lymphatic research."

The well-being of every individual depends on the health of the lymphatic system. Just imagine what that can mean in fighting a broad array of diseases. With more awareness from the medical and scientific communities and individuals like you, we can help conquer lymphedema and related lymphatic disorders, cancer, AIDS, and other illnesses. There are millions of people worldwide who currently suffer with a disorder or disease of the lymphatic system. There are also millions of others who are at risk for lymphatic malfunction such as individuals who have had surgery, injury, infection, or some other insult to their lymphatics. This list does not even include the hundreds of millions suffering from other diseases that would benefit from advances in lymphatic research and insight.

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Functions of the Lymphatic System

Fluid and Protein Balance

As blood moves through the arteries and veins, 10% of the fluid filtered by the capillaries, along with vital proteins, becomes trapped in the tissues of the body. This loss of this fluid (approximately 1-2 liters/day) would rapidly become life threatening if the lymphatic system did not properly function. The lymphatic system collects this fluid and returns it to the circulatory system.

Immunity and Spread of Infection

The lymphatic system plays an integral role in the immune functions of the body. It is the first line of defense against disease. This network of vessels and nodes transports and filters lymph fluid containing antibodies and lymphocytes (good) and bacteria (bad). The body's first contact with these invaders signals the lymphatics, calling upon this system to orchestrate the way the infection-fighting cells prevent illness and diseases from invading microorganisms.

Digestion

Lymph vessels in the lining of the gastrointestinal tract absorb fats from food. A malfunction of this part of the lymphatic system can result in serious malnutrition. The lymphatic system also impacts diseases such as excessive obesity caused by abnormal fat and carbohydrate metabolism.

Disorders of the Lymphatic System

Lymphatic Disease, Lymphedema and Related Disorders

The most prevalent lymphatic disorder is lymphatic insufficiency, or lymphedema. This is an accumulation of lymphatic fluid in the interstitial tissue causing swelling, most often in the arm(s) and/or leg(s), and occasionally in other parts of the body. The severity of this disease varies from very mild complications to a disfiguring, painful and disabling condition. In addition, patients are often susceptible to serious life-threatening cellulite infections(deep skin), and if untreated, can spread systemically or require surgical intervention. It remains a lifelong functional problem requiring daily treatment for maintenance. Eventually the skin becomes fibrotic (thickening of the skin and subcutaneous tissues) with loss of normal architecture, function and mobility.

Primary Lymphedema is an inherited condition in approximately 0.6% of live births. The lymphatic vessels are either missing or impaired and can affect from one to as many as four limbs and/or other parts of the body, including internal organs. It can be present at birth, develop at the onset of puberty or present in adulthood, with no apparent causes. Other lymphatic diseases include lipedema, cystic hygromas, lymphangiomas, lymphangiectasias, lymphangiomatosis and other mixed vascular/lymphatic malformation syndromes and conditions, such as Turner-Weber and Klippel Trenauney Syndrome.

Secondary Lymphedema (acquired regional lymphatic insufficiency) is a common problem among adults and children in the United States. It can occur following any trauma, infection or surgery that disrupts the lymphatic channels or results in the loss of lymph nodes. Among the more than 3 million breast cancer survivors alone, acquired or secondary lymphedema is believed to be present in approximately 30% of these individuals, predisposing them to the same long-term problems as described above. Lymphedema also results from prostate, uterine, cervical, abdominal, orthopedic cosmetic (liposuction) and other surgeries, malignant melanoma, and treatments used for both Hodgkin's and non-Hodgkin's lymphoma. Radiation, sports injuries, tattooing, and any physical insult to the lymphatic pathways can also cause lymphedema. Even though lymphatic insufficiency may not immediately present at the time any of the events occur, these individuals are at life-long risk for the onset of lymphedema.

Filariasis is a world health problem resulting from a parasitic-caused infection causing lymphatic insufficiency, and in some cases predisposes elephantiasis. The World Health Organization's recent efforts to eradicate the spread of infection do not address or eliminate the resulting lymphedema.

Lymphoma is a general term for a group of cancers that originate in the lymphatic system. The lymphomas are divided into two major categories: Hodgkin lymphoma and all other lymphomas, called non-Hodgkin lymphomas. Lymphomas are cancers that begin by the malignant transformation of a lymphocyte in the lymphatic system. Lymphomas, including Hodgkin lymphoma, result from an acquired injury to the DNA of a lymphocyte. Scientists know that the damage to the DNA occurs after birth and, therefore, is acquired rather than inherited. Lymphomas generally start in lymph nodes or collections of lymphatic tissue in organs like the stomach or intestines. Lymphomas may involve the marrow and the blood in some cases.

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Cancer and the Lymphatic System

The lymphatic system is critical to the body's surveillance against cancer. The lymphatic system is one of the most common avenues for the spread of cancer cells throughout the body. Science and medicine will immeasurably add to our ability to conquer cancer when we learn more about how cancer cells influence the development of new lymphatic vessels and pathways, establishing the route for these cells to spread to other parts of the body. This process known as "lymphangiogenesis" is an emerging focus within the scientific cancer research community.

Infection/HIV and the Lymphatic System

HIV and AIDS are directly connected to the lymphatic system. The ability to effectively control HIV requires increased knowledge of the lymphatics. Continued research will lead to an understanding of how infectious organisms invade the lymphatic system and overcome its normal protective role. Greater understanding of the pathways and physiology of the lymphatics, will also increase our ability to deliver antibiotic and anti-viral medication to infected tissues and organs.

Inflammation, Auto-immunity and the Lymphatic System

Inflammatory and auto-immune diseases such as rheumatoid arthritis and systemic lupus erythematosis (SLE), scleroderma, Wegener's granulomatosis, and others are impacted by the lymphatic system. All of these diseases are believed to be connected to an inflammatory process initiated by the body's immune response. The lymphatic system governs the body's immune system that normally makes the proteins, called antibodies, to protect the body against viruses, bacteria, and other foreign materials. These foreign materials are called antigens. In an autoimmune disorder such as lupus, or arthritis the immune system looses its ability to tell the difference between foreign substances (antigens) and its own cells and tissues. The immune system then makes antibodies directed against "self". These antibodies, called "auto-antibodies", react with the "self" antigens to form immune complexes. The immune complexes build up in the tissues and can cause inflammation, injury to tissues, and pain. A better understanding of how the system works will greatly increase the possibility of discovering treatments and cures for these diseases as well.

Digestion, Pulmonary Function and the Lymphatic System

Lymphatic insufficiency of the internal organs is a particularly common problem among children who are born with inherited or developmental disorders of the lymphatics. Impairment of lymphatic development in the intestines, for example, leads to malabsorption, ascites (collections of fat-laden lymph within the abdominal cavity), underdevelopment from malnutrion, immune malfunction, and premature death. Disturbed development of lymphatic channels can also exist elsewhere in the body causing serious bodily malfunction, as is the case with pulmonary lymphangiectasia, cystic hygromas and lymphangiomas. Impaired vision, swallowing and breathing difficulties are often complications resulting from these disorders.

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Scientific investigation of the lymphatic system will provide preventive and therapeutic benefits for hundreds of millions of people. The Lymphatic Research Foundation fosters and supports research to discover the links between lymphatic function, diseases, and levels of wellness.

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